This invention relates to the field of motor vehicle alternators, and more particularly to the field of temperature based controls of the output of high output capability alternators such as those found in large city busses and coaches.
Vehicle alternators with high output capability are used in large vehicles such as trucks, busses and passenger coaches. The alternator provides current for the vehicle which is used to charge the vehicle""s battery or to run various auxiliary systems. When the alternator is operating as a generator of electricity, some amount of heat is also generated by the alternator. As the current demand on the alternator is increased, the alternator will attempt to generate more electricity, thereby increasing the heat generated.
Under conventional circumstances, the alternator may be cooled by circulating oil through the alternator housing and around the internal components of the alternator. In a basic system, cool oil is pumped into the alternator. The heat generated by the internal components of the alternator is then transferred to the comparatively cooler oil, thus cooling the alternator components and heating the oil. The heated oil is then conveyed out of the alternator to a heat exchanger where the oil is cooled so that it can be recirculated to the alternator for further cooling.
In the above system, there are two separate heat exchanges occurring. In the first heat exchange, heat is transferred from the alternator to the oil. In the second heat exchange, heat is transferred from the oil to the atmosphere. In heat exchanger systems such as this, the amount of heat transferred is highly dependent on the difference in temperature between the component or fluid from which heat is being removed and the component or fluid to which heat is being moved. For vehicle based systems, the heat transferred from the oil is ultimately transferred to the air surrounding the vehicle. Consequently, the amount of heat transferred from the oil to the air, and then from the alternator to the oil, is influenced by the temperature of the ambient air. Thus, as the ambient air temperature increases, the heat transfer capacity of the cooling system decreases.
A design problem that must be addressed in vehicle alternators is that as the ambient temperature increases the use of generated electricity for some components, such as air conditioners used for the comfort of passengers, also increases. In response to this increased demand for electricity, the alternator produces more electricity and necessarily generates more heat. Consequently, as the need for heat removal from the generator increases, the system""s capacity for heat removal is decreased.
This type of system is subject to several potential failures resulting in elevated temperature of the alternator, possibly to the extent that the design temperature of the alternator is exceeded. While catastrophic failures resulting in over temperature conditions, such as a pump seizure or loss of electrical power, can occur at any time, a reduced capacity for heat removal can exacerbate otherwise nominal problems resulting in an over temperature condition. For example, the oil pump performance could become degraded, thereby limiting the amount of oil available for cooling the alternator. Additionally, the oil system could develop a leak or flow blockage restricting the amount of oil circulated through the alternator.
In the event that cooling oil flow is restricted or interrupted, the amount of heat conducted out of the alternator is reduced or eliminated. Consequently, the temperature of the alternator will increase. Should the reduced oil flow occur during periods of high current demand, the alternator temperature may exceed its maximum design operating temperature. Operating at a temperature in excess of design operating temperature can lead to stressing components beyond their design limits resulting in component failure. Depending on which component fails, the high temperature could result in an oil leak, reduced alternator output or even complete failure of the unit. Consequently, a vehicle may suffer catastrophic failure, resulting in passenger discomfort from loss of air conditioning or even stranding the passengers by complete shutdown of the vehicle. Additionally, restricted oil flow could result from a clogged oil filter which could otherwise be quickly and easily replaced at a minimal cost. The damage resulting from operating the alternator at high temperature, however, could necessitate costly and time consuming component replacement or repair.
It is readily apparent from the foregoing discussion that the severity of an alternator failure can be assessed according to two factors. The first factor is the loss of electric generating capability while the second factor is the cost of repairs. Therefore, it is desirable to provide a control system for high output capacity vehicle alternators, which minimizes operation at elevated temperatures, while avoiding complete loss of electricity generating capability and damage to the equipment.
Various devices have been used in other arts to avoid the extreme damage caused by operating electric equipment at elevated temperatures. One such device is disclosed in U.S. Pat. No. 5,546,262 issued to Baurand et al. Baurand et al. discloses a device which uses a thermistor to monitor the operating temperature of a load. A thermistor is simply a resistor which changes resistance as its temperature changes. The resulting voltage drop across the resistor is then used, typically by comparing the voltage to a reference voltage, to activate other devices. A device according to Baurand et al., in response to a high temperature condition of a load monitored by a thermistor, can interrupt power to the load thereby avoiding the catastrophic damage which could be caused by operating the load at elevated temperature.
Although the device of Baurand et al. is useful in many applications, it is of limited benefit when used to protect high capacity output alternators. As noted above, an essential factor in assessing the severity of an alternator failure is the loss of generating capability. While Baurand et al. does ameliorate the potential for damage to a piece of equipment, it does so by shutting the equipment off. This device would disable the vehicle in an over temperature condition, stranding the passengers. This is a severe shortcoming of Baurand et al. if used in conjunction with a vehicle alternator.
Baurand et al. also discloses the use of a bimetallic strip as a means for protecting an electrical load. A bimetallic strip is merely two smaller strips of metal which are joined into a single strip. A bimetallic strip operates under the principle that as the amount of current passing through the strip increases, the bimetallic strip heats up, in the same manner as any other resistor. The difference, however, is that the two metals used in a bimetallic strip expand at different rates as they are heated. Thus, the strip begins to curl as more current passes through it. At a designed current/temperature level, the strip will curl such that the electrical circuit is broken and current is no longer supplied to the load. After some amount of time, the strip cools down and returns to its original shape, thus closing the electrical circuit and current can once more be passed to the load.
The use of a bimetallic strip does mitigate damage due to operating a piece of equipment when too much current is being demanded, however, like the thermistor device of Baurand et al., a bi-metallic strip is not appropriate for use in a high capacity output alternator for a vehicle. The use of bi-metallic strips would sacrifice the operation of the alternator as a consequence of protecting the alternator from damage.
There is a significant need, therefore, to provide a control device which protects an electrical piece of equipment, such as an alternator, from over temperature conditions. Preferably, the device should not totally de-energize the electrical equipment, rather it should gradually decrease the current available to the equipment. Upon easing of the over temperature condition, the device should allow resumption of full capacity operation. The device should not require penetrations be made through the alternator housing to minimize the potential for oil leaks. The device should be easy to install on new equipment, it must also be easy to retro-fit onto existing equipment, the device should be inexpensive, comprise a minimum number of components, be relatively small, not require long leads, be compatible with other protective devices, not be subject to failure in extreme operating environments and be of simple construction.
The present invention provides a control device which protects an electrical piece of equipment, such as an alternator, from over temperature conditions. Advantageously, the present invention gradually lowers the allowed current output from an alternator in providing over-temperature protection so that there is not a total interruption of current output. Upon easing of the over-temperature condition, the invention allows resumption of full capacity current production. The invention does not require penetrations be made through the alternator housing in retrofitting or installation with new equipment, and thus, minimizes the potential for oil leaks. The invention is easy to install on new equipment, as well as easy to retro-fit onto existing equipment. Further, the invention is inexpensive, comprises a minimum number of components, is relatively small, does not require long leads, and is compatible with other protective devices, and is not subject to failure in extreme operating environments while being of simple construction.
In accordance with the present invention, a sensing means monitors the temperature of the alternator and produces an output signal representative of the sensed temperature. The output signal is passed to a variable current control device which is used to control the allowed output current from the alternator. A temperature sensor is mounted on the alternator housing to sense the temperature of the alternator. In response to an over temperature condition, a power field effect transistor is alternately energized and de-energized thus interrupting the current flow through the field winding of an alternator, thus reducing the current generated by an alternator and reducing the heat generated by the alternator.